The present invention relates to an electrophotographic photoreceptor employed in copiers, printers and the like, and more specifically to an electrophotographic photoreceptor which exhibits excellent durability.
Thirty years, and more, have passed since electrophotographic copiers were introduced onto the market. In the early years, photoreceptors, comprised of inorganic photoconductive materials such as selenium, zinc oxide, cadmium sulfide, etc., were widely employed. However, in recent years, electrophotographic organic photoconductors have been mainly employed which exhibit lower cost, non-toxicity, excellent processability, and large selection range to match specific requirements.
However, such organic photoreceptors present various problems.
Generally, in order to form an image employing an electrophotographic method, the surface of a photoreceptor is subjected to charging, image exposure, and development to form a toner image; the resulting toner image is transferred onto a transfer material, and is then fixed to obtain an image. After the transfer of the toner image, the photoreceptor is subjected to cleaning of the residual toner and discharging, and is repeatedly utilized over an extended period. Accordingly, the above-mentioned photoreceptor is required to exhibit excellent electrophotographic properties such as charge potential, dark decay potential, residual potential, etc.; excellent physical properties such as printing durability over repeated usage, abrasion resistance, moisture resistance, etc.; excellent durability against ozone generated during corona discharging and image exposure light.
Fatigue degradation of a photoreceptor caused by the repeated usage is considered to be caused by the abrasion and damage of the photoreceptor surface due to friction and adhesion of paper dust onto the surface during each process of the transfer of a toner image formed on the photoreceptor onto a transfer material, separation, and cleaning of the surface of the photoreceptor after the transfer, and furthermore, decomposition, deterioration, etc. of the photosensitive layer during each process of charging, image exposure, discharging subjected to the surface of the photoreceptor.
Accordingly, in order to minimize the fatigue degradation of the organic photoreceptor, improvements in the physical properties of the photosensitive layer are required. The photosensitive layer of the organic photoreceptor is softer than that of the inorganic photoreceptor, and because the photoconductive material is an organic one, the fatigue degradation of the photoreceptor during the repeated usage is more pronounced. Thus, improvement in the binder employed in the photosensitive layer becomes critical.
Proposed for the purpose of improving the mechanical strength of the photoreceptor, for example, are those in which a charge generating material is dispersed into a crosslinking resin such as a urethane resin (Japanese Patent Publication Open to Public Inspection No. 51-23738) and those in which a crosslinking resin is employed in a charge transport layer (Japanese Patent Publication Open to Public Inspection No. 56-48637); and further, Japanese Patent Publication Open to Public Inspection No. 56-48637 discloses a technique in which a protective layer is provided on the photosensitive layer.
Such techniques known in the art improve physical properties. On the contrary, however, some charge transport material is deposited in the charge transport layer due to insufficient compatibility between the charge transport material and the resin. This exhibits a disadvantage which adversely affects electrostatic properties such as sensitivity of the photoreceptor, residual potential, etc. Therefore, recently, a photoreceptor has been proposed which comprises a modified phenoxy resin in which physical properties are improved (Japanese Patent Publication Open to Public Inspection No. 7-160012).
According to inventions mentioned above, results to meet requirements, to some extent, have been obtained for the improvement in mechanical strength of the organic photoreceptor, and compatibility between the resin and the charge transport material. However, in recent years, the improvements in durability of the photoreceptor have been increasingly demanded, and actually, these requirements have not yet been fully met.